1. Field of the Invention
The present invention relates to an image display device for displaying an image based on visible fluorescent radiation caused by electron beams emitted from filamentary cathodes, and more particularly to such an image display device having a structure by which the filamentary cathodes are stably supported.
2. Description of the Prior Art
Cathode-ray tubes are a typical example of image display device which displays an image based on visible fluorescent radiation developed by the bombardment of a fluorescent screen by an electron beam. However, the cathode-ray tubes are necessarily long in the direction perpendicular to the fluorescent screen, and do not lend themselves to a thin, flat image display device.
Various flat image display devices have been proposed in the art. One such flat image display device comprises an array of parallel filamentary cathodes serving as a quasi-planar electron source for bombarding a patterned anode fluorescent layer with emitted electrons. According to another flat image display device, electrons emitted from an array of parallel filamentary cathodes are divided by a control electrode into a matrix of electron beams that are modulated and deflected to display an image in a matrix pattern.
Both of the above proposed flat image display devices have filamentary cathodes for emitting electron beams to cause visible fluorescent radiation for displaying an image. The flat image display devices are much thinner than the cathode-ray tubes. The filamentary cathodes held in a planar parallel configuration are required to be positioned highly accurately with respect to each other and also to the anodes and the control electrode for achieving a desired degree of displayed image quality such as image uniformity. To meet such a requirement, efforts have been made to provide a structure for supporting filamentary cathodes with high positional accuracy.
The flat image display device, with the matrix control over the electron beams for the display of an image, is additionally required to insulate the filamentary cathodes from each other so that the filamentary cathodes can be driven independently of each other.
One flat fluorescent image display device is disclosed in Japanese Laid-Open Utility Model Publication No. 57(1982)-140060, for example. FIGS. 9 and 10 of the accompanying drawings fragmentarily show the disclosed flat fluorescent image display device with respect to an arrangement for supporting filamentary cathodes.
As shown in FIGS. 9 and 10, the flat fluorescent image display device has parallel spaced filamentary cathodes 13a, 13b having ends attached to respective filament supports 14 as by welding. Lead terminals 15 have end portions shaped into a vertical wall 17 having two spaced V-shaped grooves 16 opening upwardly. The filamentary cathodes 13a, 13b have end portions received respectively in the V-shaped grooves 16 near the filament supports 14. The opposite ends (not shown) of the filamentary cathodes 13a, 13b are similarly attached and supported in position, so that the filamentary cathodes 13a, 13b are kept under tension over an array of cathodes 18. With the filamentary cathodes 13a, 13b received in and extending through the V-shaped grooves 16, the distance between the anodes 18 and the filamentary cathodes 13a, 13b and the distance between the filamentary cathodes 13a, 13b are determined by the position and dimensional accuracy of the V-shaped grooves 16 irrespective of the position and dimensional accuracy of the filament supports 14.
However, when the filamentary cathodes 13a, 13b are longitudinally elongated as they are heated in operation, the filamentary cathodes 13a, 13b are frictionally displaced in the V-shaped grooves 16. Repeated energization and de-energization of the flat image display device gradually wedge the filamentary cathodes 13a, 13b into the bottoms of the V-shaped grooves 16, frequently causing the filamentary cathodes 13a, 13b to be broken under frictional forces.
Since the V-shaped grooves 16 are required to be highly accurate with respect to their position, dimensions, and distances or pitched therebetween, the V-shaped grooves 16 are usually cut one by one in the vertical wall 17. It is highly difficult to attain a desired degree of dimensional accuracy particularly with respect to the depth of the V-shaped grooves 16. The V-shaped grooves 16 cannot easily be machined to desired shape, particularly at their bottom corners, because the tip end of a cutting tool must be sharper than the diameter (which is of about 10 .mu.m) of the filamentary cathodes 13a, 13b.
Another problem is that the filamentary cathodes 13a, 13b cannot be insulated from each other. If the vertical wall 17 were made as a separate member of a ceramic material such as alumina for electric insulation, then the cutting tool used to cut the V-shaped grooves 16 would be worn quickly, aggravating the above machining difficulties.
The filamentary cathodes 13a, 13b are generally made of tungsten so that they can be heated red. When the filamentary cathodes 13a, 13b of tungsten, as they are heated red, are held in contact with a metallic material containing nickel, the nickel is diffused into the grain boundaries of the tungsten crystal, resulting in a reduction in the mechanical strength of the filamentary cathodes 13a, 13b. The lead terminals 15 are made of a 42-6 alloy (42% Ni - 6% Cr - Fe) or a 47-6 alloy (47% Ni - 6% Cr - Fe) for desired adhesion to frit and glass and also for desired sealing capability. Therefore, the vertical wall 17, which is part of the lead terminals 15, gives rise to diffusion of nickel from the V-shaped grooves 16 into the filamentary cathodes 13a, 13b, which tends to be broken due to their reduced mechanical strength.
Japanese Laid-Open Patent Publication No. 2(1990)-121239 discloses a flat image display device in which electrons are divided into a matrix of electron beams by a control electrode. The disclosed flat image display device is shown in FIGS. 11 and 12 of the accompanying drawings. FIGS. 11 and 12 primarily show an arrangement for supporting filamentary cathodes.
As shown in FIGS. 11 and 12, each filamentary cathode 19 has opposite ends attached, as by welding, to fixed seats 20a, 20b, at least one of which is springy in nature. A back electrode 21 has a pair of electrically insulating supports 22a, 22b on its respective opposite ends, the insulating supports 22a, 22b having respective oblique grooves 23 defined therein. The filamentary cathode 19 has opposite ends portions held against surfaces of the respective grooves 23 near the fixed seats 20a, 20b. Insulating rods 24a, 24b of circular cross section are placed on the back electrode 21 near the respective ends thereof, with the filamentary cathode 19 extending over and being held against the insulating rods 24a, 24b. The insulating rods 24a, 24b are positioned outside of an image display area and inwardly of the insulating supports 22a, 22b. The filamentary cathode 19, which extends over and is held against the insulating rods 24a, 24b, is spaced from the back electrode 21 and also from a control electrode 25 positioned above the back electrode 21. The other filamentary cathodes (not shown) are also held against surfaces of other oblique grooves (not shown) defined at spaced intervals in the insulating supports 22a, 22b. Therefore, the distances between the filamentary cathodes are determined by the spaced intervals or pitches between the oblique grooves defined in the insulating supports 22a, 22b.
The insulating supports 22a, 22b by which the filamentary cathodes are insulated from each other are made of a ceramic material such as alumina since the ceramic material does not emit gases in vacuum and is capable of withstanding high temperatures. Therefore, a cutting tool used to cut the oblique grooves 23 in the insulating supports 22a, 22b is worn quickly. It is highly costly to cut the oblique grooves 23 that are required to be highly accurate in dimensions.
Since the accuracy of the spaced intervals or pitches between the filamentary cathodes 19 is governed by the surfaces of the oblique grooves 23 against which the filamentary cathodes 19 are held, the edges of the surfaces of the oblique grooves, 23 should not be beveled. When the filamentary cathodes 19 are energized and de-energized, they are longitudinally elongated and contracted, and tend to be abraded under frictional forces by the edges of the surfaces of the oblique grooves 23. Accordingly, the filamentary cathodes 19 are also apt to be broken by the abrasive action of the edges of the surfaces of the oblique grooves 23, though less frequently than by the abrasive action of the bottoms of the V-shaped grooves 16 shown in FIGS. 9 and 10.